Method of forming a particle coating on a metal component of an electric discharge device



Patented Nov. 24, 1953 METHOD OF FORMING A PARTICLE COAT- ING ON A METAL COMPONENT OF AN ELECTRIC DISCHARGE DEVICE Robert Gibson RobertshawpLondon, England, as-

signor to The General Electric Company Limited, London,'-England N0 D w ns- App on Feb a 8. 18 2 Serial No. 270,747

4 Claims.

This invention relates to electric discharge devices. More particularly, the invention is concerned with methods of forming particle coatings on metal components of electric discharge devices.

In many electric discharge devices metal com-- 'ponents are used which are provided on their surfaces with a particle coating, for example A number of methods when it is desired to provide a particle coating of small area or of very even thickness. The first requirement may arise for example in connection with the cathodes of thermionic valves designed for operation at ultra-high "frequencies,

and the second requirement may arise where the coated component is spaced a small distance from another component of the device, as may be the case with the cathode in a cathode ray tube. The known methods are also unsuitable in general where the mass of the particles is comparatively large. This is the case for example with thorium as at present produced in particle -form.

. ltis therefore an object of the invention to provide a method of forminga particle coating on a metal component of an electric discharge device, which is particularly suitable for use where the coating must be'of small area or of :very even thickness, or where the mass of the particles ,is so great as to render the known methods unsuitable.

According to the invention, a method of forming a particle coating on a metal component of an electric discharge device comprises the successive steps of preparing a suspension of finely divided particles with a binder in a solvent, pouring the suspension onto a smooth horizontal surface over which it spreads, allowing the solvent to evaporate so as to leave a thin solid film consisting of the particles and the binder, wetting an area of the surface of the metal component with a volatile solvent, applying at least a part of the film to the wetted area so that it adheres to the metal component, and heating the metal component so as to drive off the binder and so that the particles adhere firmly to the surface of the metal component.

Two arrangements in accordance with the invention will now be described by way of example, with reference to two thermionic cathodes for a 2 usein magnetron valves and provided with coatings of different electron emissive materials.

In each arrangement the cathode is directly heated and consistsof a strip of tantalum which is bent into ,a spiral so that the major faces of the strip lie on imaginary coaxial cylinders. In each case the emissive coating extends all round the outer surface of the cathode over a short distance parallel to the axis of the cathode.

In the first arrangement the emissive material used is thorium, and it is applied to the tantalum spiral as follows. Asuspension of finely divided thorium particles with a nitrocellulose binder in butyl acetate is prepared, and is poured onto a horizontal polished glass surface over which it spreads. The butyl acetate is allowed to evaporate and a thin solid film is left consisting of the thorium particles in the nitrocellulose binder. A portion of appropriate shape and size is cut from the film and is stuck to the surface of the tantalum spiral in the following manner. One end of the tantalum spiral is dipped into a quantity of acetone which rises up in the gap between adjacent turns of the spiral by capillary action and wets the surface of the tantalum strip. The portion cut from the'film is then applied to the tantalum spiral in the desired position in the following manner. The portion of the film, which is substantially flat, is placed with part of it in contact with the tantalum spiral, and then wraps itself around and adheres to the outer surface of the tantalum strip by virtue of the action of'the acetone. The portions of the film bridging the gaps between adjacent turns of the spiral are dissolved away by the acetone so that adjacent turns of the spiral are not short circuited by a conducting film. The cathode is then installed in the magnetron and during the normal processing of the valve the cathode is heated to a temperature sufiicient to cause the nitrocellulose binder to be driven oh and the thorium particles to be sintered onto the tantalum strip.

In the second arrangement the emissive material used is thoria, and in order to ensure good adherence of the thoria particle coating it is not applied directly to the tantalum strip. Instead, a tungsten particle coating is first applied to the tantalum strip over the area to be covered by the thoria coating which is then applied on top of the tungsten coating. The application of both the tungsten and thoria particle coatings is carried out in the same manner as described above with reference to the thorium particle coating, the same binder and solvents being used as before. The thoria coating is applied on top of the tungsten coating after the tungsten coating has been sintered onto the tantalum strip. It will be seen that the use of the method according to the invention allows accurate superpositioning of the thoria coating on the tungsten coating.

It will be appreciated from the foregoing description that the solvent used for preparing the original suspension need not be the same as that used for sticking the film onto the metal component. In fact, it wil1 usually be preferable to use a comparatively slow drying solvent such as butyl acetate for preparing the original suspension and a comparatively fast drying solvent such as acetone for sticking the film onto the metal component.

It will also be appreciated that the use of the invention is not restricted to the application of electron emissive coatings to thermionic cathodes as described above, but may also be used where other forms of particle coatings are to be applied to metal components of electric discharge devices. For example, the invention may be used in connection with the application of insulating particle coatings, such as alumina, to metal components of the electrode assemblies of electric discharge devices.

I claim:

1. A method of forming a particle coating on a metal component of an electric discharge device, comprising the successive steps of preparing a suspension of finely divided particles With a binder in a solvent, pouring the suspension onto a smooth horizontal surface over which. it spreads, allowing the solvent to evaporate so as to leave a thin solid film consisting of the particles and the binder, Wetting an area of the surface of the metal component with a volatile solvent for the binder, causing at least a part of the film to adhere to the metal component by applying said part of the film to the wetted area, and heating the metal component to a temperature sufficient to cause the binder to be driven off and the particles to adhere firmly to the surface of the metal component.

2. A method of manufacturing a directly heated thermionic cathode which includes a tantalum support, comprising the successive steps of preparing a suspension of finely divided thorium particles with a nitrocellulose binder in butyl acetate, pouring the suspension onto a smooth horizontal surface over which it spreads, allowing the butyl acetate to evaporate so as to leave a thin solid film consisting of the particles and the binder, wetting an area of the surface of the tantalum support with acetone, causing at 4 least a part of the film to adhere to the tantalum support by applying said part of the film to the wetted area, and heating the tantalum support to a temperature suflicient to cause the binder to be driven off and the particles to adhere firmly to the surface of the tantalum support.

3. A method of manufacturing a directly heated thermionic cathode which includes a tantalum support, comprising the successive steps of preparing a suspension of finely divided tungsten particles with a nitrocellulose binder in butyl acetate, pouring the suspension onto a smooth horizontal surface over which it spreads, allowing the butyl acetate to evaporate so as to leave a thin solid film consisting of the particles and the binder, wetting an area of the surface of the tantalum support with acetone, causing at least a part of the film to adhere to the tantalum support by applying said part of the film to the wetted area, and heating the tantalum support to a temperature suiiicient to cause the binder to be driven off and the particles to adhere firmly to the surface of the tantalum support.

4. A method according to claim 3, comprising the further successive steps of preparing a. suspension of finely divided thoria particles with a nitrocellulose binder in butyl acetate, pouring the suspension onto a smooth horizontal surface over which it spreads, allowing the butyl acetate to evaporate so as to leave a. thin solid film consisting of the particles and the binder, wetting the surface of the tungsten particle coating formed on the surface of the tantalum support with acetone, causing at least a part of the film to adhere to the tungsten particle coating by applying said part of the film to the wetted surface of the tungsten particle coating, and heating the tantalum support to a temperature suflicient to cause the binder to be driven off and the thoria particles to adhere firmly to the surface of the tungsten particle coating.

ROBERT GIBSON ROBERTSHAW.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A METHOD OF FORMING A PARTICLE COATING ON A METAL COMPONENT OF AN ELECTRIC DISCHARGE DEVICE, COMPRISING THE SUCCESSIVE STEPS OF PREPARING A SUSPENSION OF FINELY DIVIDED PARTICLES WITH A BINDER IN A SOLVENT, POURING THE SUSPENSION ONTO A SMOOTH HORIZONTAL SURFACE OVER WHICH IT SPREADS, ALLOWING THE SOLVENT TO EVAPORATE SO AS TO LEAVE A THIN SOLID FILM CONSISTING OF THE PARTICLES AND THE BINDER, WETTING AN AREA OF THE SURFACE OF THE METAL COMPONENT WITH A VOLATILE SOLVENT FOR THE BINDER, CAUSING AT LEAST A PART OF THE FILM TO ADHERE TO THE METAL COMPONENT BY APPLYING SAID PART OF THE FILM TO THE WETTED AREA, AND HEATING THE METAL COMPONENT TO A TEMPERATURE SUFFICIENT TO CAUSE THE BINDER TO BE DRIVEN OFF AND THE PARTICLES TO ADHERE FIRMLY TO THE SURFACE OF THE METAL COMPONENT. 